TUESDAY, June 9, 2015 (HealthDay News) -- Researchers are launching a clinical trial to see if a vaccine approved long ago to prevent tuberculosis may also hold promise as a treatment for type 1 diabetes.

The proposed five-year study is designed to investigate whether repeated injections of the tuberculosis vaccine bacille Calmette-Guerin (or BCG vaccine) can quiet the immune system attack that causes type 1 diabetes and improve blood sugar levels in people with long-standing diabetes.

"BCG is showing so much promise in worldwide trials [for conditions such as multiple sclerosis]," said study author Dr. Denise Faustman, director of the immunobiology laboratory at Massachusetts General Hospital in Boston.

While some may hold out hope that BCG will reverse type 1 diabetes in people, findings from Faustman's earlier -- albeit smaller -- human trial suggest the effects are likely to be far more subtle.

"The goal [for the new study] is to create a therapeutic response," said Faustman, who added that such a response might help prevent some of the most serious complications of type 1 diabetes.

The researchers announced the start of their phase 2 trial Sunday at the American Diabetes Association annual meeting in Boston.

Type 1 diabetes is an autoimmune disease. That means the body's immune system mistakenly attacks a healthy part of the body as if it were a foreign substance. In the case of type 1 diabetes, the immune system turns on the insulin-producing beta cells in the pancreas.

The BCG vaccine works by increasing levels of a substance called tumor necrosis factor (TNF) in the body, Faustman said. Higher levels of TNF cause an increase in the amount of good immune system cells, and lower levels of the bad cells that are responsible for destroying the beta cells, she explained.

Higher levels of TNF appear to be helpful in certain autoimmune diseases, such as multiple sclerosis, celiac disease and possibly some forms of psoriasis and autoimmune thyroid disease, according to Faustman. There are, however, some autoimmune conditions -- such as rheumatoid arthritis -- where higher TNF levels can be a problem.

But in type 1 diabetes, higher TNF levels appear to lower the attack on the beta cells. And the BCG vaccine causes increased amounts of TNF. Once the immune system attack is dampened, it appears that the pancreas can regenerate at least some insulin-producing beta cells, Faustman said.

She noted that the BCG vaccine has been used to prevent tuberculosis infections for about 90 years, so it has a long record of safety.

The phase 1 trial of the vaccine included six people with type 1 diabetes. The average time the study volunteers had diabetes was 15 years. They were randomly assigned to receive two injections of the vaccine or a placebo. That study lasted 20 weeks.

Two out of three people given the vaccine showed evidence that the vaccine increased the good immune cells and decreased the bad immune cells. The researchers also saw evidence of insulin production.

The new randomized, double-blind trial will include 150 adults between the ages of 18 and 60. Faustman is looking for people who've had type 1 diabetes for a long time, probably around 15 to 20 years. They have to still have some activity in their pancreas. This can be measured with a blood test.

Volunteers will receive two injections, of either the vaccine or a placebo, two weeks apart. Then they'll be given a single injection annually for the next four years. Faustman said initially they'll need to do blood tests every two weeks or so. Eventually, blood tests will only need to be done every six months to once a year, she said.

Although the trial will be done in Boston, Faustman said people don't have to live in the Boston area to be part of the study.

Dr. Joel Zonszein, director of the clinical diabetes center at Montefiore Medical Center in New York City, said, "It would be very nice if there is any type of response to the BCG vaccine. The vaccine is safe, and it's cheap."

But Zonszein said he has his doubts. "The body is very smart. The mechanisms in the body have so many redundancies, I'm reluctant to believe that selective immunosuppression would reverse type 1 diabetes."